Method and system for tunable pulsed laser source

ABSTRACT

A tunable pulsed laser source comprising a seed source adapted to generate a seed signal and an optical circulator. The optical circulator includes a first port coupled to the seed source, a second port, and a third port. The laser source also includes an amplitude modulator characterized by a first side and a second side. The first side is coupled to the second port of the optical circulator. The laser source further includes a first optical amplifier characterized by an input end and a reflective end including a spectral-domain reflectance filter. The input end is coupled to the second side of the amplitude modulator. Moreover, the laser source includes a second optical amplifier coupled to the third port of the optical circulator.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/737,052, filed on Apr. 18, 2007, entitled “Method and System forTunable Pulsed Laser Source,” which claims benefit under 35 U.S.C.§119(e) of U.S. Provisional Patent Application No. 60/793,307, filedApr. 18, 2006, entitled “Method and System for Tunable Pulsed LaserSource.” The contents of both above-referenced applications are herebyincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of tunable lasersources. More particularly, the present invention relates to a methodand apparatus for providing high power pulsed laser sources useful forindustrial applications such as trimming, marking, cutting, and welding.Merely by way of example, the invention has been applied to a lasersource with real-time tunable characteristics including pulse width,peak power, repetition rate, and pulse shape. However, the presentinvention has broader applicability and can be applied to other lasersources.

Pulsed laser sources, such as Nd:YAG lasers have been used to performlaser-based material processing for applications such as marking,engraving, micromachining, and cutting. Depending on the application andthe materials to be processed, the various characteristics of the laserpulses, including pulse width, pulse repetition rate, peak power orenergy, and pulse shape, are selected as appropriate to the particularapplication. Many existing high power pulsed lasers, for example, havepulse energy greater than 0.5 mJ per pulse) rely on techniques such asQ-switching and mode locking to generate optical pulses. However, suchlasers produce optical pulses with characteristics that arepredetermined by the cavity geometry, the mirror reflectivities, and thelike and cannot generally be varied in the field without compromisingthe laser performance. Using such lasers, it is generally difficult toachieve a range of variable pulse characteristics.

Thus, there is a need in the art for pulsed laser sources with tunablepulse characteristics.

SUMMARY OF THE INVENTION

According to the present invention, techniques related generally to thefield of tunable laser sources are provided. More particularly, thepresent invention relates to a method and apparatus for providing highpower pulsed laser sources useful for industrial applications such astrimming, marking, cutting, and welding. Merely by way of example, theinvention has been applied to a laser source with real-time tunablecharacteristics including pulse width, peak power, repetition rate, andpulse shape. However, the present invention has broader applicabilityand can be applied to other laser sources.

According to an embodiment of the present invention, a tunable pulsedlaser source is provided. The tunable pulse laser source includes a seedsource adapted to generate a seed signal and an optical circulator. Theoptical circulator includes a first port coupled to the seed source, asecond port, and a third port. The tunable pulse laser source alsoincludes an amplitude modulator characterized by a first side and asecond side. The first side is coupled to the second port of the opticalcirculator. The tunable pulse laser source further includes a firstoptical amplifier characterized by an input end and a reflective endincluding a spectral-domain reflectance filter. The input end is coupledto the second side of the amplitude modulator. Moreover, the tunablepulse laser source includes a second optical amplifier coupled to thethird port of the optical circulator.

According to another embodiment of the present invention, a method ofproviding one or more laser pulses is provided. The method includesproviding a seed signal at a first port of an optical circulator,transmitting the seed signal to the first side of an amplitudemodulator, and transmitting the seed signal through the amplitudemodulator to define a first pass through the amplitude modulator. Themethod also includes time-domain filtering the seed signal to provide apulse. Time-domain filtering includes modulating a drive signal for theamplitude modulator. The method further includes amplifying the pulseusing a first optical amplifier and frequency-domain filtering theamplified pulse to provide a spectrally filtered pulse. Additionally,the method includes transmitting the spectrally filtered pulse throughthe amplitude modulator to define a second pass through the amplitudemodulator and time-domain filtering the amplified spectrally filteredpulse to provide an intermediate pulse. Time-domain filtering includesmodulating the drive signal for the amplitude modulator. Moreover, themethod includes amplifying the intermediate pulse using a second opticalamplifier.

Numerous benefits are achieved using the present invention overconventional techniques. For example, in an embodiment according to thepresent invention, high power, pulsed lasers suitable for laserprocessing are provided that utilize a compact architecture that isinexpensive in comparison to lasers with comparable performancecharacteristics. Moreover, in embodiments of the present invention,short pulses are generated with pulse characteristics that are tunablein real-time while maintaining pulse-to-pulse stability. Furthermore, inan embodiment according to the present invention, optical pulses can beshaped to optimize the pulse profile for the particular application, orto maximize energy extraction efficiency in the laser system. Dependingupon the embodiment, one or more of these benefits may exist. These andother benefits have been described throughout the present specificationand more particularly below. Various additional objects, features andadvantages of the present invention can be more fully appreciated withreference to the detailed description and accompanying drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic illustration of a high power pulsedlaser with tunable pulse characteristics using optical fiber amplifiersaccording to an embodiment of the present invention;

FIG. 2 is a simplified timing diagram illustrating electrical andoptical pulses at different locations in a high power pulsed laseraccording to an embodiment of the present invention; and

FIG. 3 is a simplified illustration of a method of providing a series oflaser pulses according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 is a simplified schematic illustration of a high power pulsedlaser with tunable pulse characteristics using optical fiber amplifiersaccording to an embodiment of the present invention. High power pulsedlaser 100 includes a seed source 110 that generates a seed signal thatis injected into a first port 114 of an optical circulator 120.According to an embodiment of the present invention, the optical seedsignal is generated by using a seed source 110 that is a continuous wave(CW) semiconductor laser. In a particular embodiment, the CWsemiconductor laser is a fiber Bragg grating (FBG) stabilizedsemiconductor diode laser operating at a wavelength of 1032 nm with anoutput power of 20 mW. In another particular embodiment, the CWsemiconductor laser is an external cavity semiconductor diode laseroperating at a wavelength of 1064 nm with an output power of 100 mW. Theoutput power may be lower or greater than 100 mW. For example, theoutput power can be 50 mW, 150 mW, 200 mW, 250 mW, or the like. Inalternative embodiments, the seed signal is generated by a compactsolid-state laser or a fiber laser. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives.

After passing through the optical circulator 120, the seed signal exitsfrom a second port 122 of the circulator 120 and impinges on a firstside 132 of an optical amplitude modulator 130. Circulators are wellknown in the art and are available from several suppliers, for example,model OC-3-1064-PM from OFR, Inc. of Caldwell, N.J.

The optical amplitude modulator 130 is normally held in an “off” state,in which the signal impinging on the modulator is not transmitted.According to embodiments of the present invention, optical amplitudemodulator provides amplitude modulation and time-domain filtering of theseed signal as well as amplified spontaneous emission (ASE) filtering.In a particular embodiment, the length of the optical pulse isdetermined by the operation of the optical amplitude modulator 130,which may be an APE-type Lithium Niobate Mach-Zehnder modulator having abandwidth >3 GHz at 1064 nm.

According to embodiments of the present invention, the optical amplitudemodulator 130 is an electro-optic Mach-Zehnder type modulator, whichprovides the bandwidth necessary for generating short optical pulses. Inother embodiments, the optical amplitude modulator 130 is a phase orfrequency modulator with a suitable phase or frequency to amplitudeconverter, such as an edge optical filter, an extinction modulator, oran acousto-optic modulator. For example, an electro-optic phasemodulator can induce a frequency chirp to the optical signal, whichwould be converted into an amplitude modulation when the optical signalis transmitted through a short or long pass optical filter. Preferably,the optical signal would be characterized by a wavelength thatexperiences high loss when no electrical signal is applied to theelectro-optic phase modulator. When an electrical signal is applied tothe electro-optic phase modulator, the optical signal preferablyexperiences a change in wavelength or frequency chirp to a valuecharacterized by low optical loss.

In order to pass the seed signal, the optical amplitude modulator 130 ispulsed to the “on” state for a first time to generate an optical pulsealong optical path 136. The pulse width and pulse shape of the opticalpulse generated by the optical amplitude modulator 130 are controlledvia by the modulator drive signal applied to the optical amplitudemodulator 130. The optical pulse then passes for a first time through afirst optical amplifier 150, where it is amplified. According toembodiments of the present invention, the amplitude modulator, driven bya time varying drive signal, provides time-domain filtering of the seedsignal, thereby generating a laser pulse with predetermined pulsecharacteristics, including pulse width, pulse shape, and pulserepetition rate.

According to an embodiment of the present invention, the opticalamplifier 150 is an optical fiber amplifier. Fiber amplifiers utilizedin embodiments of the present invention include, but are not limited torare-earth-doped single-clad, double-clad, or even multiple-clad opticalfibers. The rare-earth dopants used in such fiber amplifiers includeYtterbium, Erbium, Holmium, Praseodymium, Thulium, or Neodymium. In aparticular embodiment, all of the fiber-optic based components utilizedin constructing optical amplifier 150 utilize polarization-maintainingsingle-mode fiber.

Referring to FIG. 1, in embodiments utilizing fiber amplifiers, a pump142 is coupled to a rare-earth-doped fiber loop 144 through opticalcoupler 140. Generally, a semiconductor pump laser is used as pump 142.One of ordinary skill in the art would recognize many variations,modifications, and alternatives. In alternative embodiments, the opticalamplifier 150 is a solid-state amplifier including, but not limited to,a solid-state rod amplifier, a solid-state disk amplifier or gaseousgain media.

In a particular embodiment, the optical amplifier 150 includes a 5 meterlength of rare-earth doped fiber 144, having a core diameter ofapproximately 4.1 μm, and doped with Ytterbium to a doping density ofapproximately 4×10²⁴ ions/m³. The amplifier 150 also includes a pump142, which is an FBG-stabilised semiconductor laser diode operating at awavelength of 976 nm, and having an output power of 100 mW. The outputpower can be lower or greater than 100 mW. For example, it can be 50 mW,150 mW, 200 mW, 250 mW, 300 mW, 350 mW, 400 mW, or the like. In anotherparticular embodiment, the pump 142 is a semiconductor laser diodeoperating at a wavelength of about 915 nm. In yet another particularembodiment, the pump 142 is a semiconductor laser diode operating at anoutput power of 450 mW or more. In a specific embodiment, the amplifier150 also includes a pump to fiber coupler 140, which is a WDM pumpcombiner.

The signal emerging from optical amplifier 150 along optical path 148then impinges on a reflecting structure 146, and is reflected back intooptical amplifier 150. The signal passes for a second time throughoptical amplifier 150, wherein the signal is amplified. The reflectingstructure 146 performs spectral domain filtering of the laser pulse andof the amplified spontaneous emission (ASE) propagating past opticalpath 148. Thus, the seed signal experiences both amplitude andtime-domain modulation passing through amplitude modulator 130, andspectral-domain filtering upon reflection from reflecting structure 146.

In an embodiment, the reflecting structure 146 is a fiber Bragg grating(FBG) that is written directly in the fiber used as the opticalamplifier 150. The periodicity and grating characteristics of the FBGare selected to provide desired reflectance coefficients as is wellknown in the art. Merely by way of example in a particular embodiment,the reflecting structure 146 is a FBG having a peak reflectance greaterthan 90%, and a center wavelength and spectral width closely matched tothe output of the seed source 110.

The signal emerging from optical amplifier 150 along optical path 136impinges on the second side 134 of the optical amplitude modulator 130,which is then pulsed to the “on” state a second time to allow theincident pulse to pass through. According to embodiments of the presentinvention, the timing of the second “on” pulse of the optical amplitudemodulator 130 is synchronized with the first opening of the modulator130 (first “on” pulse) to take account of the transit time of the signalthrough the amplifier 150 and the reflecting structure 146. Afteremerging from the first side of the optical amplitude modulator 130, theamplified pulse then enters the second port 122 of optical circulator120, and exits from the third port 116 of optical circulator 120 alongoptical path 148.

The signal is then amplified as it passes through a second opticalamplifier 160. As discussed in relation to FIG. 1, embodiments of thepresent invention utilize a fiber amplifier as optical amplifier 160,including a pump 154 that is coupled to a rare-earth-doped fiber loop156 through an optical coupler 152. Generally, a semiconductor pumplaser is used as pump 154, although pumping of optical amplifiers can beachieved by other means as will be evident to one of skill in the art.In a particular embodiment, the second optical amplifier 160 includes a5 meter length of rare-earth doped fiber 156, having a core diameter ofapproximately 4.8 μm, and is doped with Ytterbium to a doping density ofapproximately 6×10²⁴ ions/m³. The amplifier 160 also includes a pump154, which is an FBG-stabilised semiconductor laser diode operating at awavelength of 976 nm, and having an output power of 500 mW. In anotherparticular embodiment, the second optical amplifier 160 includes a 2meter length of rare-earth doped fiber 156, having a core diameter ofapproximately 10 μm, and is doped with Ytterbium to a doping density ofapproximately 1×10²⁶ ions/m³. The fiber length can be shorter or longerthan 2 meters. For example, it can be 1.0 m, 3.0 m, 3.5 m, 4.0 m, 4.5 m,5.0 m, or the like. The amplifier 160 can also include a multimode pump154, which is a semiconductor laser diode having an output power of 5 W.The output power can be lower or greater than 5 W. For example, it canbe 3 W, 4 W, 6 W, 7 W, 8 W, 9 W, 10 W, or the like.

In another particular embodiment, in order to pass the seed signal, theoptical amplitude modulator 130 is pulsed once instead of twice. Theoptical amplitude modulator 130 is turned to the “on” state to generatethe rising edge of the pulse propagating along optical path 136. Thissignal is then amplified a first time through optical amplifier 150. Thesignal then impinges on the reflecting structure 146 and is amplified asecond time through optical amplifier 150. Now the signal emerging fromoptical amplifier 150 along optical path 136 impinges on the second side134 of the optical amplitude modulator 130, which is subsequently turnedto the “off” state. The pulse width is therefore given by the timeduration during which the optical amplitude modulator 130 is held in the“on” state subtracted by the transit time of the signal through theamplifier 150 and the reflecting structure 146.

Although FIG. 1 illustrates the use of a single optical amplifier 160coupled to the third port of the optical circulator 120, this is notrequired by the present invention. In alternative embodiments, multipleoptical amplifiers are utilized downstream of the optical circulator 120as appropriate to the particular applications. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

FIG. 2 is a simplified timing diagram illustrating electrical andoptical pulses at different locations in a high power pulsed laseraccording to an embodiment of the present invention. Merely by way ofexample, FIG. 2 illustrates the timing of repetitive electrical drivesignals to the amplitude modulator and optical pulses propagatingthrough an embodiment of the invention as described in FIG. 1. Followingan electrical trigger 210, a first electrical drive signal 220 isapplied to the amplitude modulator to generate an optical pulse 240.After some propagation delay, the optical signal 250 passes through theoptical amplifier a first time. The optical signal 260 then impinges onthe reflecting structure and passes through the optical amplifier asecond time 250. The optical pulses 240 are transmitted through theamplitude modulator a second time, which is driven electrically a secondtime 220 with the optical pulses 240. Finally the optical pulses 230exit port 3 of the circulator after some propagation delay.

FIG. 3 is a simplified illustration of a method of providing a series oflaser pulses according to an embodiment of the present invention. Themethod includes providing a seed signal (310). In embodiments of thepresent invention, the seed signal can be generated by a semiconductorlaser at a wavelength of 1064 nm. The method also includes thetransmission of the seed signal through an amplitude modulator (320) fora first time. In embodiments of the present invention, the coupling ofthe seed signal into the amplitude modulator can be facilitated by anoptical circulator or other means of optical coupling. The methodfurther provides for time-domain filtering of the seed signal byapplying a drive signal to the amplitude modulator a first time (330).The pulse is amplified by the optical amplifier (340) andfrequency-domain filtered (350). Thus, a spectrally filtered pulse isprovided at one stage of the system illustrated in FIG. 3.

It will be appreciated that several combinations of amplifiers andfrequency-domain filtering architectures can be utilized withoutdeparting from the scope of the embodiments described herein. Forexample, the frequency-domain filtering can be achieved before or afteramplification of the pulse. Additionally, frequency-domain filtering canbe achieved after a first amplification of the pulse and before a secondamplification of the pulse as would happen in a double-pass opticalamplifier. Moreover, the method includes transmitting the optical signala second time through the amplitude modulator (360) and providingtime-domain filtering of the pulsed signal by applying a drive signal tothe amplitude modulator a second time (370). After passing through theamplitude modulator a second time, the amplified spectrally andtemporally filtered pulse may be referred to as an intermediate pulse.In embodiments of the present invention, the optical pulse is generatedby modulating the seed signal during a first-pass transmission throughthe amplitude modulator and is then gated during a second-passtransmission through the amplitude modulator.

Utilizing embodiments of the present invention, high power pulsed lasersources are provided that generate streams of optical pulses withindependently adjustable pulse characteristics including pulse width,peak power and energy, pulse shape, and pulse repetition rate. Merely byway of example, a particular embodiment of the present inventiondelivers output pulses at the output 170 of second optical amplifier 160of more than 5 μJ per pulse at a pulse width of 10 ns and at arepetition rate of 10 kHz. Of course, other pulse characteristics areprovided by alternative embodiments. For example, the pulse energy canbe 1 μJ, 10 μJ, 20 μJ, 30 μJ, or the like. The pulse duration is in arange of values, for example, between 2 ns and 150 ns. The pulserepetition frequency is in a range of values, for example, 0 to 500 kHz.

In the embodiments described above, a CW seed source is utilized andtime-domain filtering to provide a laser pulse is performed using theamplitude modulator 120. However, this is not required by the presentinvention. In an alternative embodiment, the seed signal is modulated toprovide a pulsed seed signal rather than CW seed signal. Providing apulsed seed signal minimizes parasitic signal build-up caused by seedleakage and enables the operating power range of the seed source to beincreased. For example, the pulse seed output power can be 200 mW, 500mW, or even 1 W. In this alternative embodiment, the pulsed seed signalmay be of a pulse width equal to, or longer than the desired pulse widthof overall pulsed laser source. Pulsing the seed can also increase theeffective linewidth of the seed laser to reduce Stimulated BrillouinScattering (SBS).

According to embodiments of the present invention, methods and systemsare provided that result in the generation of sequences of opticalpulses, which may not be equally separated in time. Moreover, the pulsewidths and pulse energies are individually tailored in a predeterminedmanner from pulse to pulse. Furthermore, it will be recognized thatalthough the above description discussed the generation of a singleoptical pulse, embodiments of the present invention provide for thegeneration of multiple pulses by repeating the single pulse amultiplicity of times. These multiple pulses may include an arbitrarytrain of optical pulse sequences.

While the present invention has been described with respect toparticular embodiments and specific examples thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention. The scope of the invention should, therefore, bedetermined with reference to the appended claims along with their fullscope of equivalents.

1. A tunable pulsed laser source comprising: a seed source characterizedby a seed wavelength; an optical circulator including a first portcoupled to the seed source, a second port, and a third port; anamplitude modulator characterized by a first side and a second side,wherein the first side is coupled to the second port of the opticalcirculator; a first optical amplifier characterized by an input end andan output end, wherein the input end is coupled to the second side ofthe amplitude modulator; and a reflective component coupled to theoutput end of the first optical amplifier.
 2. The tunable pulsed lasersource of claim 1 further comprising a second optical amplifier coupledto the third port of the optical circulator.
 3. The tunable pulsed lasersource of claim 1 wherein the reflective component comprises a fiberBragg grating (FBG) having a constant grating period.
 4. The tunablepulsed laser source of claim 3 wherein the FBG is characterized by apeak reflectance at the seed wavelength of greater than 90%.
 5. Thetunable pulsed laser source of claim 1 wherein the amplitude modulatorcomprises a Mach-Zehnder interferometric amplitude modulator adapted toperform time-domain transmission filtering of the seed signal.
 6. Thetunable pulsed laser source of claim 1 wherein the first opticalamplifier comprises a double-pass fiber amplifier.
 7. A tunable pulsedlaser source comprising: a seed source adapted to generate a seedsignal; an optical circulator including a first port coupled to the seedsource, a second port, and a third port; an amplitude modulatorcharacterized by a first side and a second side, wherein the first sideis coupled to the second port of the optical circulator; a first opticalamplifier characterized by an input end and an output end, wherein theinput end is coupled to the second side of the amplitude modulator; anda fiber Bragg grating (FBG) coupled to the output end of the firstoptical amplifier.
 8. The tunable pulsed laser source of claim 7 whereinthe FBG comprises a chirped FBG characterized by a variation in gratingperiod.
 9. The tunable pulsed laser source of claim 8 where thevariation in grating period is linear.
 10. The tunable pulsed lasersource of claim 8 where the variation in grating period is exponential.11. The tunable pulsed laser source of claim 7 further comprising asecond optical amplifier coupled to the third port of the opticalcirculator.
 12. The tunable pulsed laser source of claim 7 wherein thefirst optical fiber amplifier comprises a double-pass fiber amplifier.13. The tunable pulsed laser source of claim 7 wherein the first opticalamplifier comprises a rare-earth doped optical fiber.
 14. A tunablepulsed laser source comprising: a seed source characterized by a seedwavelength and a seed signal spectral bandwidth; an optical circulatorincluding a first port coupled to the seed source, a second port, and athird port; an amplitude modulator characterized by a first side and asecond side, wherein the first side is coupled to the second port of theoptical circulator; a first optical amplifier characterized by an inputend and an output end, wherein the input end is coupled to the secondside of the amplitude modulator; and a reflective structure coupled tothe output end of the first optical amplifier.
 15. The tunable pulsedlaser source of claim 14 wherein the reflective structure comprises afirst fiber Bragg grating (FBG) and a second FBG optically coupled tothe output end of the first optical amplifier, wherein the first FBG isseparated by a first distance from the output end of the first opticalamplifier and the second FBG is separated by a second distance from theoutput end of the first optical amplifier.
 16. The tunable pulsed lasersource of claim 15 wherein the first FBG is characterized by a peakreflectance at a first wavelength and the second FBG is characterized bya peak reflectance at a second wavelength.
 17. The tunable pulsed lasersource of claim 16 wherein the first wavelength and the secondwavelength are within the seed signal spectral bandwidth.
 18. Thetunable pulsed laser source of claim 16 wherein a peak reflectance atthe first wavelength and the second wavelength is greater than 90%. 19.The tunable pulsed laser source of claim 14 further comprising a secondoptical amplifier coupled to the third port of the optical circulator.20. The tunable pulsed laser source of claim 14 wherein the firstoptical amplifier comprises a rare-earth doped optical fiber.